1. Field of the Invention
The present invention relates to a video recorder with a distortion corrector circuit.
2. Description of the Prior Art
From the German patent specification DE-3809394 there is known a distortion corrector circuit arranged in the signal path for processing the frequency-modulated brightness signals of a video recorder and containing a band-pass filter that is tuned to the deflection range of the part of the brightness signal that actually reproduces the brightness. This band-pass filter consists of a filter circuit component located in the signal path and a coupling component arranged downstream thereof. A diode switch is also connected in parallel with the output of the filter circuit component. When the oscillation amplitudes of the frequency-modulated brightness signal at the output side of the filter circuit component exceed a switching voltage of the diode switch, the said voltage being defined in the vicinity of the flection point of the diode switch characteristic, the diode switch will pass from a blocked state (off-state) to a conducting state (on-state) in this part of the characteristic and, together with the filter circuit component and the conducting state d.c. resistance of the diode switch, will form a transit-time controller with an amplitude-dependent retardation of tile transit time of the frequency-modulated brightness signal. In this switching state of the distortion corrector circuit the transit time of the frequency-modulated brightness signals is therefore retarded in a manner proportional to the amplitude of their oscillations, so that these signals have their transit time corrected. This transit time retardation compensates frequency and phase frequency characteristic errors, especially in the lower-frequency range of the lower sideband of the carrier frequencies of the frequency-modulated brightness signal immediately adjacent to the range of variation of the carrier oscillations, so that the amplitude-modulated brightness signals at the output side of the frequency modulator will maintain a linear frequency characteristic up to very high video frequencies (beyond 3 MHz), thereby making it possible to attain a very high image resolution in the image reproduction from the videotape without any additional noise. On the other hand, given small oscillation amplitudes of the transmitted frequency-modulated brightness signals that do not exceed the threshold value of the diode switch and therefore maintain the diode switch in its off-state, so that the entire distortion corrector circuit will be in the state of bandpass filter operation, the resonance effects of the band-pass filter, which to all intents and purposes trigger the basic or carrier oscillations of the frequency-modulated brightness signal, will make themselves felt. This leads to the suppression of the low oscillations of the lower sideband of the frequency-modulated brightness signals, the said frequencies having superposed on them the carrier frequencies of the frequency-modulated brightness signals and being capable--given adequate amplitudes in the vicinity of their maximum amplitude--of taking the carrier frequencies of the frequency-modulated brightness signal out of the switching range of the frequency modulator. Furthermore, this will also increase the amplitude of the carrier oscillations themselves and reduce the noise signals in the frequency range above the carrier oscillations.
These measures ensure that a sharper image will be reproduced on the screen. Furthermore, the arrangement substantially eliminates the edge noises that disturb image reproduction, especially the noises due to vertical brightness edges, as well as disturbing reflections and the defects described as "flitter".
However, the practical application of these measures has shown that the quality of the recording of video information on the tape of a video cassette and the quality of the video tapes themselves are so widely divergent as to make it impossible to find a design or dimensioning of the known distortion corrector circuit that can cover in a satisfactory manner this wide range of differences in quality of the video cassettes to be reproduced.